Unlike thin-film transistor liquid-crystal (TFT-LCD) display devices which need a backlight system for light emission, active matrix organic light-emitting diode (AMOLED) display devices emit light by themselves and are therefore more visible, brighter and thinner. Presently, AMOLED display devices are regarded as a new generation of display devices that will replace TFT-LCD display devices.
Each pixel element in an AMOLED display device includes a pixel circuit which serves mainly to provide an organic light-emitting diode (OLED) with a stable current. Reference is now made to FIG. 1 for the basic configuration of a pixel circuit in an AMOLED display device of the prior art. As shown in FIG. 1, the conventional pixel circuit 10 comprises: a thin film transistor (TFT), an OLED and a storage capacitor Cs. An output D of the TFT is coupled to an input of the OLED, and two terminals of the storage capacitor Cs are respectively coupled to the input and an output of the OLED. During operation of the pixel circuit 10, a pixel-side positive power supply voltage ELVdd and a pixel-side negative power supply voltage ELVss are input to a first input S of the TFT and the output of the OLED, respectively, concurrently with a data voltage Vdata being input to a second input G of the TFT. Based on the difference between the pixel-side positive power supply voltage ELVdd and the data voltage Vdata, the TFT produces a driving current which drives the OLED to emit light. The storage capacitor Cs acts to stabilize the current flowing through the OLED.
Thus, in the conventional pixel circuit, the brightness of the OLED and hence the light emission and grayscale performance of the corresponding pixel are controlled by the TFT serving as a voltage/current converter and the capacitor acting as a signal storage unit. As the brightness of the OLED is proportional to the current flowing therein, desirable light emission of it can be guaranteed if the current is maintained at a fixed level. The driving current, which determines the brightness of the OLED, is in turn determined by the difference between the pixel-side positive power supply voltage ELVdd and the data voltage Vdata.
However, in practical application, it has been found that OLEDs in AMOLED display devices fall short of their expected brightness levels due to gamma shifts, which is detrimental to display quality. In order to improve the AMOLED display quality, those skilled in this art have been looking for the causes and solutions for the occurrence of gamma shifts in them.